1. Field of the Invention
The present invention relates to computer data storage systems. More particularly, the invention relates to magnetic media computer data storage systems.
2. Background and Summary
In magnetic tape systems, information is recorded on the tape by magnetizing magnetic particles impregnated into the tape. This is accomplished through a record head that contains a piece of magnetic material with a coil of wire around it that forms an electromagnet. The information, which one wishes to store, is then applied to the record head in the form of an electrical signal that changes the magnetic field of the electromagnet. Thus, as the tape is passed relative to the record head, the information or data is stored on the tape by magnetizing the aforementioned particles on the tape as the tape passes by.
Data is retrieved from the tape in a similar manner. A tape which has the magnetic impregnated particles magnetized in a fashion directly related to the data is passed by a playback head. Again, a playback head is typically a coil of wire around a magnetic material forming an electromagnet. Depending on the use, the record and playback heads may be one and the same. Thus, as the tape is passed by the head a magnetic field is induced into the magnetic material of the playback head which in turn creates an electrical signal in the coil of wire which is directly related to the data stored on the tape. Obviously, the placement of these heads is important to the storage and retrieval of the information on the tape.
The placement of the heads becomes more important as the desire of the industry for small media storage devices with higher storage capacity increases. Open format tape storage technologies such as the Linear Tape-Open (xe2x80x9cLTOxe2x80x9d) format are directed toward meeting this desire. Specifically the LTO format, provides four separate data bands separated laterally across the width of the tape with five servo bands bracketing the data bands. The data in these servo bands are offset longitudinally along the axis of travel of the tape by specified amounts to allow the tape unit to identify which data band the data heads are positioned over by measuring the timing offset between the upper and lower servo bands bracketing each data band. The longitudinal offsets between adjacent servo bands are +33.33, xe2x88x9233.33, +66.66 and xe2x88x9266.66 (xcexcm) micrometers. Thus, the encoding of the timing data in the servo bands in this format must be placed with a high degree of accuracy to meet the desired longitudinal offsets.
The LTO format specifies multiple servo positions within each servo band. The number of servo positions depends on the implementation. The distance between two adjacent servo positions corresponds to the distance between the data bands. The space between bands written in opposing directions is called a direction buffer. This space is designed to minimize interference between bands written in opposite directions that arise from variations in tape guiding and servo performance. Open tape formats, such as LTO, combine the advantages of linear multi-channel bi-directional tape formats with improvements in timing based servo control, hardware data compression, optimized band layouts, and efficient error correction routines to maximize capacity and performance. The LTO tape format specifications include the ACCELIS and ULTRIUM specifications. The ACCELIS specification provides a 8 (mm) millimeter wide tape on a dual reel cartridge with high speed access to stored data on two data bands. The ULTRIUM tape format provides a xc2xd inch wide tape with ultra-high-capacity storage on four data bands.
Tape head movement in the LTO format is controlled by the servo system using information encoded in servo bands bordering both sides of each data band. Two types of information are encoded into the servo bands. The first type of information provides a cross-tape position error signal using the robust Timing Based Servo (TBS). The second type of information provides absolute down-the-tape longitudinal positioning information, which allows the tape to span and maneuver along the length of the band for precise data handling operations. The absolute location down the length of the tape is recorded in longitudinal position (LPOS) marks information encoded into the TBS servo code.
FIG. 1 depicts a representation of the layout of the data bands and servo bands for the LTO format. As shown, there are four data bands each of which is bordered by two adjacent servo bands per the LTO specifications.
With reference to FIG. 2, the previous approach to encoding the aforementioned servo bands onto tape prior to use for data storage was done in the following manner. The head 40 consisted of one large core 20 with a single winding 10. The gaps 30 in the head were then precision machined to give the appropriate longitudinal offsets 50. The previous approach suffers in several respects. First, the task of manufacturing a head with the requisite precision in the placement of the gaps is far from trivial. In fact, failures are common resulting in the heads being discarded due to imprecise placement in the machining of the gaps. Second, since the gaps are distributed longitudinally across the surface of the head, the contact area of the tape head interface is rather large. This ultimately limits the maximum speed that the media can achieve while maintaining proper head to tape contact, which limits the rate at which servo data may be encoded. Finally, in order to achieve the requisite magnetic flux from each of the distributed gaps, the inductance of the single winding in the core must be relatively large. Thus, the head presents a highly inductive load that is difficult for the associated signal electronics to drive, especially as frequency increases. This makes it difficult to achieve a low rise time on the data signal applied to the head. Also, as part of this, any variation in the machining of the gaps or the core resulting in the magnetic flux density emerging from the gaps being unequal is unable to be compensated for due to the common drive signal.
The present invention overcomes the aforementioned disadvantages as well as other disadvantages. In accordance with the teachings of the present invention, a write head having multiple independent servo write cores aligned vertically with reference to the magnetic tape media is provided. All the servo write cores preferably have independent drivers and data generators, allowing them to encode data independently on a magnetic tape media. Each servo write core driver is preferably independently clocked from a precision source so it can be offset with respect to the other write core drivers in precise small steps.